This invention relates to in situ recovery of mineral values, and more particularly to techniques for controlling subsidence of overburden adjacent the boundary of a development region containing a plurality of in situ recovery zones. More particularly it relates to controlling subsidence adjacent the boundary of a region containing a plurality of in situ oil shale retorts each containing a fragmented mass of formation particles in which oil shale is retorted for recovering shale oil.
U.S. patent application Ser. No. 878,387, filed Feb. 16, 1978, entitled IN SITU OIL SHALE RETORTS WITH GAS BARRIERS FOR MAXIMIZING PRODUCT RECOVERY, by Irving G. Studebaker and Ned M. Hutchins, and assigned to the same assignee as this application, now U.S. Pat. No. 4,176,882, describes a technique for in situ oil shale retorting wherein the load of overburden is shared by gas barriers and "fragmented masses" so that rupture of the overburden above the gas barriers is prevented. The present application concerns a transition zone between such a development and an adjacent region having overburden supported by unfragmented formation. The aforementioned patent application is incorporated herein by this reference.
The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil form kerogen in the oil shale deposits. It should be noted that the term "oil shale" as used in the industry is in fact a misnomer; it is neither shale, nor does it contain oil. It is a sedimentary formation comprising marlstone deposit with layers containing an organic polymer called "kerogen", which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein, and the liquid hydrocarbon product is called "shale oil".
A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, such as U.S. Pat. Nos. 3,661,423; 4,043,585; 4,043,596; 4,043,597 and 4,043,598, which are incorporated herein by this reference. Such patents describe in situ recovery of liquid and gaseous hydrocarbon materials from a subterranean formation containing oil shale by fragmenting such formation to form a stationary, fragmented permeable body or mass of formation particles containing oil shale within the formation, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products, thereby producing retorted oil shale.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-containing retort inlet mixture into the retort as an oxygen-supplying gaseous combustion zone feed to advance the combustion zone through the retort. In the combustion zone, oxygen in the combustion zone feed is depleted by reaction with hot carbonaceous materials to produce heat, combustion gas, and combusted oil shale. By the continued introduction of the retort inlet mixture into the retort, the combustion zone is advanced through the fragmented mass in the retort.
The combustion gas and the portion of the combustion zone feed that does not take part in the combustion process pass through the fragmented mass in the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition, called retorting, in the oil shale to gaseous and liquid hydrocarbon products, and to a residual solid carbonaceous material.
The liquid products and gaseous products are cooled by the cooler oil shale fragmented in the retort on the advancing side of the retorting zone. The liquid hydrocarbon products, together with water produced in or added to the retort, are collected at the bottom of the retort. An off gas containing combustion gas, including carbon dioxide generated in the combustion zone, gaseous products produced in the retorting zone, carbon dioxide from carbonate decomposition, and any gaseous retort inlet mixture that does not take part in the combustion process, is also withdrawn from the bottom of the retort. The products of retorting are referred to herein as liquid and gaseous products.
Residual carbonaceous material in the retorted oil shale can be used as fuel for advancing the combustion zone through the retorted oil shale. When the residual carbonaceous material is heated to its spontaneous ignition temperature it reacts with oxygen. As the residual carbonaceous material becomes depleted in the combustion process, the oxygen penetrates farther into the oil shale retort where it combines with remaining unoxidized residual carbonaceous material, thereby causing the combustion zone to advance through the fragmented oil shale.
It is desirable to maximize the amount of oil shale subjected to retorting within a region of formation being developed. To this end it is desirable to minimize the amount of formation excavated from each retort site when forming void volumes in preparation for explosive expansion. The mined out formation is excluded from the in situ retorting process, which can reduce the overall recovery of shale oil from the retorts. Removed formation either must be retorted by above ground techniques, or the shale oil is lost when the mined out material is discarded. Moreover, the steps of mining the shale and transporting it to above ground are expensive and time consuming.
When forming a group of in situ retorts, substantial amounts of unfragmented formation are left in the vertical partitions or barriers between adjacent fragmented masses in the group of retorts. The partitions or barriers between individual retorts contain essentially unrecoverable shale oil, but such barriers are left in place because they serve as gas barriers which make it possible to independently control retorting operations in each fragmented mass within the group of retorts, and they substantially prevent leakage of off gas into adjacent underground workings where operating personnel may be present.
During retorting, a substantial amount of the kerogen present in the partitions of unfragmented formation which provide such gas barriers is not retorted. Therefore, to maximize the yield from a group of in situ retorts, it is desirable to form gas barriers which are as thin as possible so they contain the least practical amount of kerogen while still being sufficiently thick that they can inhibit the flow of gases between adjacent retorts.
A retort development region having relatively thin gas barriers formed between retorts can experience subsidence of overburden above the retorts. When load from the overburden is carried by the fragmented masses following explosive expansion, some crushing of the particles at the particle interfaces occurs, which can result in subsidence of formation in the retort development region. Subsidence of overburden does not occur outside the retort development region where overburden is supported completely by the unfragmented formation. For example, in an exemplary embodiment where a group of retorts are formed within an outside boundary of a retort development region, about 75 to 80% of the horizontal area of formation within the development region is occupied by the fragmented masses, and about 20 to 25% of the area is in the gas barriers of unfragmented formation between the fragmented masses. Ground subsidence can occur over the development region since the overburden is supported largely by fragmented formation. Outside the boundary of the development region, where overburden is supported only by unfragmented formation, no ground subsidence occurs.
An abrupt change in subsidence can occur at the boundary of a development region having a transition between a region that has subsided and a region that is still completely supported by unfragmented formation. Such an abrupt change can cause formation to rupture along a generally vertical shear plane between the subsided region and an adjacent nonsubsided region which is completely supported by unfragmented formation. Rupture of formation along such a shear plane is to be avoided because it can cause leakage of water from overlying aquifers into retort or mining areas, leakage of gas from completed retorts, leakage of air into retort during retorting operations, and safety hazards in underground workings containing operating personnel.
Similar effects can occur in other in situ recovery techniques where a fragmented mass remains undergound. For example, it can be desirable to form a subterranean recovery zone containing a fragmented mass of ore containing metal values and then recover such metal values from the fragmented mass. A leach liquid can be percolated through such a fragmented mass for extracting metal values. Such a technique of in situ leaching can be used alone or in combination with in situ retorting of oil shale.
Thus, it is desirable to avoid abrupt changes in subsidence near the boundaries of an in situ development region containing a plurality of subterranean recovery zones, each containing a fragmented mass of particles.